Summonable Creatures will become Available after you beat the Goldor and free your ship. Once you defeat Goldor, release your ship from its chains and fly to the top of the North Western Continent. Do not under ant circumstances fly over the castle in the area as you will be shot down. Once there you will find a village call Reptilio and will be able to buy summonable characters from the magic shop. Escape is the cheapest one at 100 Gill.